The primary purpose of a control valve is to control the flow of a fluid, such as steam, gas, water, and the like. Positioning a moveable operator, for example a valve plug, against a valve seat within the control valve body controls fluid flow. As the valve plug moves proximate to the valve seat, a variable orifice is created that can modulate or control an amount of fluid passing through the valve body. Under certain operational conditions, such as when the valve plug is in contact with the valve seat, leakage of the fluid may still occur. The American National Standards Institute (“ANSI”) has established leakage classifications for control valves according to a valve's ability to shut off flow when the valve is closed. ANSI specifies different leakage classes, such as ANSI Leakage Class I, II, III, IV, and V, relating to the amount of flow allowed to pass through the valve when the valve is in a closed position. In general, the leakage requirements become more stringent proceeding from Class I to Class V and therefore more difficult for a valve to meet and maintain these requirements. Specifically, ANSI Leakage Class V states that the maximum leakage allowed through a valve is 0.0005 ml of water per minute, per inch of port diameter, per PSI differential pressure as measured from an inlet port of the valve to an outlet port of the valve. For example, a control valve with 2-inch diameter inlet and outlet ports, with 100 PSI of pressure applied to a fluid passing through the valve, can have up to 0.1 ml of leakage per minute and still satisfy Leakage Class V requirements.
Moreover, conventional control valves with large port sizes are designed to balance the force applied to the valve plug, in an effort to reduce the thrust and therefore the size of the actuator used to operate the valve. The reduction in actuator thrust limits the amount of force produced to seat the valve plug against the valve seat. Consequently, such valves have difficulty meeting the strict Class V requirements, while concurrently providing a balanced valve plug.
FIG. 1 is a cross-sectional illustration of a conventional double port valve that will not meet Class V leakage requirements. Double port valves are often utilized to balance net forces acting on a valve plug and to minimize an actuator force necessary to position the valve plug. The illustrated conventional double port valve 10 has a valve body 10A assembly (valve body) that is generally coupled to an actuator (not shown). The actuator is typically a pneumatically powered device that supplies the force and motion to open or close a valve. The valve body 10A houses a valve plug 11 that is coupled to a valve stem 18. The valve stem 18 is in turn coupled to an actuator stem (not shown) that transmits the actuator thrust to move the valve plug 11. The value plug is positioned within the flow path of the fluid and is movable to selectively modify the rate of flow through the valve. The illustrated value plug has two lands, a first land 12 and a second land 13. The beveled surfaces of the lands 12 and 13 form sealing surfaces when they matingly engage a seat ring 14 at an upper port, and a seat ring 15 at a lower port, respectively.
The surface areas of the first land 12 and the second land 13 are generally of unequal size. When the valve plug 11 is inserted through the top of the valve body 10A during valve assembly, the second land 13 must be sized to pass through the seat ring 14. The first land 12, however, is relatively larger than the second land 13, such that the first land 12 does not pass through the seat ring 14. Rather, the first land 12 seats against the seat ring 14 to form a seal. The difference in size between the first land 12 and the second land 13 results in the surface area of the first land 12 being relatively larger than the second land 13.
In addition to meeting the requirements of a particular ANSI Leakage Class, there is often a desire for the valve plug to be balanced, so that there is a minimal amount of force necessary to open and close the valve with the actuators. Inside a valve, the valve plug moves to block or open a fluid passageway through the valve between the valve inlet and the valve outlet. When the valve plug is shut off (i.e., the valve is closed) there can be a fluid pressure pushing against the valve plug from the inlet side of the valve. The fluid pressure results from the pressurized fluid on the inlet side that is blocked by the valve plug from passing through the valve. The fluid, therefore, pushes on the valve plug.
More specifically, the larger surface area of the first land 12 relative to the second land 13 provides more surface area for fluid to act upon within the valve. This results in the fluid pressure from the inlet 16 exerting a relatively greater force on the first land 12 than on the second land 13 during shut off conditions. The resulting net force is in a direction that opposes the closing of the valve 10 (e.g., upwards in FIG. 1). This net force must be overcome by additional actuator thrust to close the valve 10. Any added force acting against the actuator results in an increased load on the actuator, and can lead to requirement of the larger actuator. Conversely, in the conventional double port design, the surface areas of the first land 12 and the second land 13 are relatively similar in total area. As such, the net force exerted on the valve plug 11 is relatively small.
In addition, it is relatively difficult to machine the lands 12 and 13 and seat rings 14 and 15 to the relatively tight tolerances necessary to prevent leakage at shut off, and to maintain thermal expansion differences at a minimum. Consequently, the example double port valve 10 is difficult to manufacture in a manner such that it would meet the maximum leakage requirements of ANSI Leakage Class V at higher operating temperatures.
In a valve employing a balanced valve plug arrangement, the valve plug most often moves in a direction perpendicular to the fluid flow. However, one of ordinary skill in the art will appreciate that other valve configurations, including angle valves, can have similar characteristics to those described herein. The chambers within the valve body are arranged such that there is fluid on both ends of the valve plug. This results in a first fluid pressure acting against valve plug movement at one end, and a second fluid pressure acting with valve plug movement at the other end. Therefore, fluid pressure forces tend to oppose one another, ensuring that resistance to opening and closing the valve is negligible. The forces on either side of the valve plug are greatly influenced by the port areas against which the internal fluid pressures in the valve act. Thus, when the net forces (the result of the first fluid force subtracted from the second fluid force) acting on the ends of the valve plug are minimized, the size of the actuator is minimized, resulting in a more cost effective valve solution.
FIG. 2 is a cross-sectional illustration of a double seat valve 20 with a balanced valve plug designed to meet the leakage requirements of ANSI Leakage Class V. The valve 20 has a valve plug 21 positioned by a valve stem 22. The valve stem 22 is in turn coupled to an actuator (not shown). The valve stem 22 protrudes from the valve 20 through a bonnet 23. The bonnet 23 includes a valve packing 24 that provides a fluid seal and serves to guide the valve stem 22 within the valve 20. As depicted, the valve plug 21 has a two-part construction, which includes a pilot plug 25 and a primary plug 26. The primary plug 26 cooperates with openings 27 formed in a cage 28 and with a lower seat ring 31 to control the flow of fluid between an inlet port 29 and an outlet port 30. The cage 28 forms part of a valve trim assembly that surrounds the valve plug 21 and helps characterize the flow of fluid passing through the valve. The valve trim typically modulates the fluid flow. The primary plug 26 is lifted away from the seat ring 31 by a washer 32 attached to the end of the pilot plug 25 by a nut 33. The primary plug 26 is biased toward the seat ring 31 by a plurality of springs, such as springs 37 and 38, forming a fluid seal.
The pilot plug 25 includes openings 35 and 36, which allow the fluid pressures above and below the pilot plug 25 to equalize. Thus, when the valve 20 is to be opened, the forces acting on the pilot plug 25 caused by fluid pressure on either side of the pilot plug 25 are relatively balanced. The valve stem 22 lifts the pilot plug 25 away from an upper valve seat 34, which is formed in the primary plug 26. This allows the pressure on both sides of the primary plug 26 to equalize through openings 39 formed in the primary plug 26.
When the valve 20 is to be closed, the actuator drives the valve stem 22, which in turn drives the primary plug 26 against the seat ring 31 to form a fluid seal. The springs 37 and 38 exert a force between the pilot plug 25 and the primary valve plug 26 to keep the port between the pilot plug 25 and the upper valve seat 34 open. This maintains the pressures across the primary plug 26 in a relatively balanced state until the primary plug 26 is seated against the lower valve seat 31. When the primary plug 26 is seated against the lower valve seat 31, the valve stem 22 causes the pilot plug 25 to seat against the upper valve seat 34, completing closure of the valve 20. This double seat arrangement allows the valve 20 to achieve Class V shut off.
Drawbacks of this double seat valve construction are that it is relatively complicated, and requires expensive, precisely machined components to achieve this balanced design. In addition, pilot-plug valve designs can become unstable in certain operating conditions.